Add __setitem__ slicing

[bgrant]

Building on #384 and #385, this PR allows slice syntax in a DistArray's __setitem__.

The same restrictions apply: BlockMaps and NoDistMaps only and slice-steps of size 1 only.

[kwmsmith]

I presume the value_slices[arr.comm_rank] is necessary because we can't scatter the slices to the targets?

[kwmsmith]

What about doing a for loop of self.apply() calls, each iteration sending the slice to its target?

[bgrant]

Wouldn't that force us to wait on n serialized roundtrips though?

[bgrant]

And apropos scattering: yes.

[kwmsmith]

Some strange behavior:

>>> c = Context()
>>> dist = Distribution.from_shape(c, (10, 20))
>>> da = c.ones(dist)
>>> db = da[:5, :10]
>>> db * 2
[2:apply]:
---------------------------------------------------------------------------
Traceback (most recent call last)<string> in <module>()
[...]
AttributeError: 'DummyMod' object has no attribute '__distarray__b4369fc35ada6cd50'

[3:apply]:
(same error)

This looks like the operation is being called on targets where the sliced array doesn't exist.

Also:

>>> arr = db.tondarray()
>>> db[...] = arr * 2
>>> db.tondarray()
array([[ 2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.],
       [ 2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.],
       [ 2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.],
       [ 2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.],
       [ 2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.,  2.]])

That looks good, but:

>>> db[...] = db * 2
[2:apply]:
[...]
AttributeError: 'DummyMod' object has no attribute '__distarray__b4369fc35ada6cd50'

[3:apply]:
(same error)

[coveralls]

Coverage increased (+0.49%) when pulling 2d75aa4 on feature/setitem-slicing into de18b91 on master.